The present invention relates to a coating apparatus for coating uniformly a magnetic coating, a metallic coating or a resin solution or the like on a polyester base or on a sheet-shaped object such as a thin metallic sheet, without any coating defect such as streak defect and uneven and nonuniform coating layer, and more particularly, to a coating apparatus which is used preferably for manufacturing magnetic recording media.
Magnetic recording media are obtained by coating a magnetic coating solution on a support and drying it, and as an apparatus for coating a magnetic coating solution on a support, a roll coater, a gravure coater and an extrusion coater are generally used. In the case of the extrusion coater among them, an excellent coating thickness can be obtained even in the case of high speed coating such as the coating speed of 200 m/min or more or of thin layer coating such as the wet coating thickness of 10 .mu.m or less. However, the extrusion coating has problems that white clearness on a coated layer and streak defects are easily caused.
One of the reasons for these defects is that foreign materials, dust or coagulations stick to an edge surface (hereinafter referred to as a back edge surface) positioned at the extremely downstream side of the extrusion coater during coating. As disclosed in Japanese Patent Publication Open to Public Inspection Nos. 207865/1990 and 207866/1990, therefore, there are suggested methods wherein adhesion of extraneous matters on the back edge surface of an extrusion coater is prevented by making the value of center line mean roughness (Ra) on the back edge surface of the extrusion coater small.
Following the densification of magnetic recording media, the coating thickness of a magnetic layer to be coated on a support is required to be thinner in the manufacturing process. Further, the coating speed is in a trend for higher speed from the viewpoint of productivity, and thereby, enough time for leveling can not be given to a coating solution, thus, the suggestion disclosed in the aforementioned official documents, namely, only to make the value of center line mean roughness on the back edge surface of the extrusion coater small has become insufficient for preventing coating defects such as uneven and nonuniform coating layer and streak defects.